Catalyst systems

ABSTRACT

A system and method for the after-treatment of exhaust gases from an internal combustion engine is provided. The system includes a first catalyst disposed downstream of the engine, through which the exhaust gases flow and which is effective to catalyze a shift reaction between carbon monoxide and water, or which is exhaust gas. The system also includes an oxygen source disposed between the first catalyst and a hydrocarbon trap, and downstream from all of those a three-way catalyst to catalytically oxidize hydrocarbons and carbon monoxide to reduce noxious emissions.

FIELD OF INVENTION

The present invention concerns improvements in catalyst systems for thecontrol of exhaust gas emissions from vehicles.

BACKGROUND OF INVENTION

The emissions from vehicle exhausts are now well known to causepollution, health problems and ecological damage. For this reason,various governmental or quasi-governmental bodies have issuedregulations giving maximum levels of the main pollutants, namely carbonmonoxide (CO), unburnt hydrocarbons (HC), NOx and, in the case of dieselengines, particulates. Increasing standards have been published, to comeinto effect at various times in the future, with the eventual aim forexample in the USA to meet demanding regulations known as “ULEV” (UltraLow Emission Vehicle), with a proportion of vehicles having zeroregulated emissions “ZEV”. A variety of strategies have been suggested,including a system described in WO 96/39576 (Johnson Matthey) which hasbeen found in tests in cars to exceed the ULEV standards. This systemprincipally uses a very low light-off catalyst, which starts conversionof CO and/or H₂ immediately upon start-up of the engine, thus creatingan exotherm which rapidly raises catalyst temperature to the point of HClight-off. (“Light-off” is understood to be the temperature at which agiven amount, for example 50% by weight, of a reactant is converted.)

There remains, however, the need for a system that is robust and able tocope with the wide variety of gasoline grades marketed, especially withregard to high sulphur levels and other possible catalyst poisons,especially traces of lead. The above-mentioned WO 96/39576 states thatit is preferred to use CO as a “fuel” for initiating HC light-off. It isacknowledged that most engines do not produce significant quantities ofhydrogen in the exhaust, and therefore a secondary source of hydrogen,such as an on-board reformer, would be necessary if hydrogen were to berequired to play a major part in speeding HC light-off. A reformeritself requires an appreciable time to start-up to produce hydrogen.Another feature of the described development is that it does not requireso-called “starter” catalysts mounted in the “close-coupled” positionvery close to, or even within, the exhaust gas manifold. WO 93/18346(South West Research) discloses the use of a first combustion chamber inan internal combustion engine to produce an exhaust gas which is treatedby a water gas shift reactor to produce a gas enriched in hydrogen, andthereafter recycling that hydrogen-enriched gas to another combustionchamber, to reduce the overall emissions of unburnt hydrocarbons andnitrogen oxides. All additional hydrogen produced is consumed within theengine.

GB 2,277,045 (Ford Motor Co.) concerns an adoption of the “EGI” (ExhaustGas Ingnition) system which utilises an afterburner, and requires thetemporary trapping of unburnt hydrocarbons so that only CO and hydrogenreach an afterburner where the gases are ignited by a spark plug. It isto be noted that this disclosure requires the use of an afterburnerseparate from a catalytic convertor downstream of the afterburner. Thisdisclosure does not contemplate either increasing the hydrogen contentof the exhaust gases or utilising hydrogen except in an EGI system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a motor vehicle incorporating acatalyst system according to the present invention;

FIG. 2 shows SCAT rig data for the generation of hydrogen by threedifferent catalyst samples;

FIG. 3 shows test results for an engine operated in accordance with theinvention and illustrates carbon monoxide decay and increased hydrogenconcentrations;

FIG. 4 shows SCAT test results where the hydrogen/carbon dioxide ratiowas manipulated to illustrate different degrees of water gas shiftcatalyst performance;

FIG. 5 shows light-off temperature of a conventional three-way catalystwhere no upstream starter catalyst is used;

FIG. 6 shows temperature plots at the inlet and the outlet of the TWC;and

FIG. 7 shows results from a complete system in accordance with thepresent invention.

DETAILED DESCRIPTION OF INVENTION

The present invention provides a system for the after treatment of theexhaust gases from a gasoline-fuelled internal combustion enginedesigned to operate under essentially stoichiometric conditions,comprising a first catalyst through which the exhaust gases flow andwhich is effective to catalyse a shift reaction between CO and H₂O or iseffective to catalyse the conversion of a reactant to generate ahydrogen-enriched exhaust gas, a source of gaseous oxygen, a trap forhydrocarbons and a three-way catalyst effective to catalytically oxidiseHC and CO and to reduce NO_(x).

The invention further provides a method of reducing pollutants from agasoline-fuelled internal combustion engine designed to operate underessentially stoichiometric conditions, particularly during start-up,comprising starting the engine under rich conditions, enriching theresulting exhaust gases with hydrogen by a catalytic shift reaction,diluting the resulting exhaust gases with gaseous oxygen to produce leanconditions, trapping hydrocarbons in a hydrocarbon trap, and initiatingoxidation of the hydrogen and CO over a three-way catalyst.

The present invention yields unexpected benefits, in that the presenceof hydrogen is not only beneficial from the consideration of theexotherm from hydrogen oxidation causing more rapid heating of thethree-way catalyst to reach light-off temperature for HC, but alsoresults in the surprising reduction of HC light-off temperature. Thisaspect could not have been predicted before the present invention.Accordingly, although we do not wish to be restricted by any theoryexpressed herein, it is possible in preferred embodiments to trap HCupon start-up and delay the desorption of HC from the trap until thethree-way catalyst has reached the reduced light-off temperature for HCoxidation. This latter intention has been expressed in the past, but hasproved impossible to achieve. The invention desirably includes controlmeans to initiate and terminate the addition of the appropriate quantityof gaseous oxygen upon start-up of the engine. Suitable air pumps areavailable to the skilled person. It will be appreciated that a richerstart-up, for example at an air-fuel ratio (“λ”) of 0.8, whilst commonbefore emissions regulations, is generally thought to be undersirablebecause of the increased levels of HC at start-up, that is before anycatalyst can begin to convert HC. State-of-the-art starting strategiestend to be “lean-start”, but this does require careful control and ismore difficult to achieve than a rich-start strategy.

The first catalyst is preferably optimised to generate hydrogen usingthe shift reaction, under rich conditions. Desirably, the first catalystis in the close-coupled position, located within or not more than about20-30 cm from the exhaust gas manifold. This ensures that the catalystimmediately receives the benefit of the combustion heat from the engine,so that the generation of hydrogen is practically immediate, thusavoiding start-up delays from reformers. The first catalyst may be quitesmall, as is customary for “start-up” catalysts, and is desirablysupported on a metal or ceramic honeycomb support of conventional type.The catalyst itself is desirably a platinum group metal catalyst, eg oneor more of Pd, Rh and Pt, more desirably Pt or Rh. The catalyst supportis desirably coated with a high surface area washcoat of, for examplestabilised alumina and/or a ceria-zirconia mixed oxide or a ceria,preferably the latter. The catalyst may be manufactured according tomethods generally known in the art. If desired, the catalyst may bepromoted. Desirable loadings of precious metal are 1.77 to 14.12 g/liter(50 to 400 g/cuft), preferably 3.53 to 10.59 g/liter (100 to 300 g/cuft)of catalyst volume. It is preferred that the catalyst is selective tothe production of H₂ and CO₂ rather than CH₄.

As an alternative to the above-desired shift reaction catalyst, areagent may be added to the exhaust gases prior to the first catalyst,which reagent generates hydrogen under the typical start-up conditions.For example, the injection of methanol can generate appreciable volumesof hydrogen, and 7 to 8 ml of methanol is considered sufficient to yieldthe benefits of the present invention. Other reagents may be used.

The exhaust gases leaving the first catalyst are enriched with hydrogen.In order to ensure that the highly exothermic oxidation of hydrogen onthe second catalyst takes place to best effect, it is required tosupplement the oxygen also present in the exhaust gases, suitably by asecondary air addition. Such a secondary air addition is desirablyimmediately before the second catalyst, suitably by a connection intothe exhaust pipework not more than about 10 cm before the three-waycatalyst. The secondary air is suitably provided by a pump, which may beelectrically powered. The pump is desirably activated on start-up of theengine and may usefully be controlled by the electronic engine controlunit. According to the individual engine design, the characteristics ofthe exhaust gas, and other contributing factors including the three-waycatalyst composition, the secondary air may be operated for a limitedperiod upon start-up of the engine, or may be operated continuously atthe same or a lower air addition after light-off of the HC oxidationprocess. In a further embodiment, the secondary air addition may beadditionally activated in an intermittent manner upon detection orapproximation of an operating condition that requires supplementary air,for example in the event of high sulphur levels causing poisoning of thethree-way catalyst.

Suitably HC traps are generally available in the art, and suitablycomprise a deposit of a zeolite carried on a through-flow honeycombcatalyst support, which may be separate from or integral with thesupport used for the three-way catalyst. Desirably, the HC trap ismounted immediately before the three-way catalyst, especially within thesame metal “box” or “can”.

The three-way catalyst is desirably another platinum group metalcatalyst, desirably comprising one or more of Pd, Rh and Pt, moredesirably Pt or Pt/Rh. The catalyst is preferably supported on awash-coated metal or ceramic through-flow honeycomb of conventionalconstruction, suitably having 15.5 to 93 or more cells/sq cm (100 to 600or higher cells/sq in.). Suitable loadings of PGM catalyst are 1.77 to14.12 g/liter (50 to 400 g/ft³) catalyst volume.

The three-way catalyst is conveniently a conventional catalyst; thesehave been found by extensive usage to be technically robust andresistant to degradation. This is in some contrast to catalystsdeveloped to have light-off temperatures below ambient as required bythe above-mentioned WO 96/39,576, which often show sensitivity tosulphur. Conventional three-way catalysts are also relativelyinexpensive.

It is desirable that the engine management (ECU=engine control unit) isprogrammed to maximise the potential for hydrogen generation during thestart-up period. This may be achieved not only by control of air/fuelratio, λ, but also in some engines by valve and/or ignition timing.Initial tests have shown that the CO content of the exhaust gases uponstart-up can be increased to almost 10% by volume. Our studies indicatethat a hydrogen content of approximately 5 to 7% by vol should meet atleast our initial targets on reaching light-off temperatures, but higherhydrogen contents, e.g. up to 20% by vol, would offer advantages underat least some conditions.

The present invention is illustrated by reference to the accompanyingdrawings, in which FIG. 1 is a schematic diagram of a motor vehicleincorporating a catalyst system according to the invention.

A gasoline engine, 1, has an exhaust system, 2, comprising exhaustmanifold, 3, and exhaust pipe, 4. (Conventional silencer box or boxesare not shown.) A first catalyst, 5, is mounted close to the connectionof the manifold to the pipe, 4. The pipe 4 is connected to an underfloorcatalytic convertor box, 6, and a secondary air entry, 7, is locatedimmediately upstream of the convertor box 6. The secondary air isprovided by a pump, 8, controlled by the engine control unit, 9. Withinthe box 6 is a hydrocarbon trap, 10, mounted immediately upstream of athree-way catalyst, 11. The box is connected to the vehicle tailpipe,12.

When the engine is cranked to initiate starting, the air pump 8 isactivated. The exhaust gases, which are initially rich in HC and CO,contact the first catalyst 5 and CO and H₂O are converted to H₂ and CO₂,thus enriching the exhaust gases with hydrogen. The enriched gases thenpass to the catalytic convertor box after being dosed with secondaryair, typically to create lean conditions equivalent to λ=1.1.Hydrocarbons are trapped in trap 10 as soon as the exhaust gases contactit, and are not desorbed until the trap is heated to an appropriatetemperature. CO and H₂ are oxidised on the three-way catalyst, and theexotherm generated is sufficient to raise the temperature of thecatalyst to in excess of 150° C. within a few seconds. At this point,the hydrocarbons begin to desorb and can be immediately oxidised overthe catalyst, since the presence of hydrogen has reduced the light-offtemperature for hydrocarbons. Such speed of response is not believed tobe available through competing technologies including electricallyheated catalysts, whether active or passive, except possibly some systemincorporating a pre-heating device; such devices require considerablepower and connection to external electrical power sources withconsiderable additional system equipment costs.

The invention is illustrated further by reference to a number of tests.Two types of experimental data are presented below. The first is derivedfrom a synthetic catalyst test (SCAT) rig, which tests small catalystcores using gas mixtures generated from bottled gas supplies. The secondsource is an engine test bed, fitted with a 4-cylinder, 1.8 litergasoline engine of current design technology.

Test 1 illustrates SCAT rig data for the generation of hydrogen by threedifferent catalyst samples, one of which is a commercial standardstarter catalyst composition (1) having 100 g/cuft total Pd+Rh in a 14:1wt ratio, carried on alumina/ceria with base metal promoters, the second(2) has 300 g/cuft Pd carried on the same alumina/ceria support, andcatalyst (3) has 300 g/cuft Rh carried on ceria but with alumina as abinder. The considerable increase in the quantity of hydrogen generatedcan easily be seen. It should be noted that the test actually measuredCO₂ which is easier to measure than H₂ but is prepared in identicalvolumes according to the equation CO+H₂O→CO₂+H₂. The synthetic gasmixture comprises 7% carbon monoxide, 8% water vapor and 85% nitrogen,which is used to represent engine-like rich start emissions (but with nohydrocarbon and nitrogen oxides components). The results are shown inthe attached drawing “Test 1”.

Test 2 illustrates actual engine results, where the engine is operatedat 3,000 rpm and the exhaust flow rate is ramped from 50 to 73 kg/hour.Here the decay in post-catalyst carbon monoxide and the increase inhydrogen gas concentrations demonstrate the performance of the startercatalyst (3) selected after Test 1 in a practical application. Startercatalyst (3) is a close-coupled catalyst (“CCC”).

Test 3 illustrates SCAT rig results where the hydrogen/carbon dioxideratio is manipulated to illustrate different degrees of water gas shiftcatalyst performance. Here it is shown that a high hydrogen contentenhances the light-off of a conventional three-way catalyst.

Test 4 illustrates a typical light-off temperature of a conventionalthree-way catalyst, as mounted on the test bed engine discussedpreviously. Here no upstream starter catalyst is fitted. Light-off ofthe TWC occurs at 155° C.

Test 5 illustrates temperature plots at the inlet and the outlet of theTWC. Both the starter and the three-way catalyst are fitted. Here thehydrogen generated by the upstream starter catalyst significantly lowersthe light-off temperature of the three-way catalyst, to 135° C.

Test 6 (Example of the invention) illustrates results from the completesystem, where the hydrogen generating starter catalyst, hydrocarbon trapand three-way catalyst are fitted. Here further improvements in thelight-off temperature of the three-way catalyst are illustrated This isbelieved to occur because the hydrocarbons adsorbed by the trap wouldnormally suppress the three-way catalyst light-off. The light-offtemperature is further reduced to 115° C.

The skilled person can readily modify the invention as described toobtain the benefits conceived, without departing from the scope of thepresent invention.

What is claimed is:
 1. A system for the after-treatment of the exhaustgases from a gasoline-fuelled internal combustion engine designed tooperate under essentially stoichiometric conditions, comprising a firstcatalyst disposed downstream of said engine, through which the exhaustgases flow and which is effective to catalyse a shift reaction betweenCO and H₂O or is effective to catalyse the conversion of a reactant togenerate a hydrogen enriched exhaust gas; a source of gaseous oxygen; atrap for hydrocarbons; and a three-way catalyst effective tocatalystically oxidise HC and CO and to reduce NO_(x).
 2. A systemaccording to claim 1, wherein said first catalyst comprises at least oneof Pt and Rh.
 3. A system according to claim 1 comprising an enginecontrol unit effective to initiate, terminate or limit the quantity ofgaseous oxygen supplied.
 4. A system according to claim 3, wherein saidfirst catalyst comprises at least one of Pt and Rh.
 5. A systemaccording to claim 1, comprising an engine control unit programmed togenerate above-average amounts of CO in the exhaust gases during astart-up portion of the engine operating cycle.
 6. A system according toclaim 5, wherein the engine control unit is effective to initiate,terminate or limit the gaseous oxygen supplied.
 7. A system according toclaim 5, wherein said first catalyst comprises at least one of Pt andRh.
 8. A method of reducing pollutants from a gasoline-fuelled internalcombustion engine, during start-up, wherein the engine is designed tooperate after start-up under essentially stoichiometric conditions,which method comprises starting the engine under rich conditions,enriching the resulting exhaust gases with hydrogen by a catalytic watergas shift reaction to produce a hydrogen enriched gas, diluting theresulting exhaust gases with gaseous oxygen to produce lean conditions,trapping hydrocarbons in a hydrocarbon trap, and initiating oxidation ofthe hydrogen and CO over a three-way catalyst.
 9. A method of reducingpollutants from a gasoline-fuelled internal combustion engine whereinthe engine is designed to operate after start-up under essentiallystoichiometric conditions, which method comprises starting the engineunder rich conditions, enriching the resulting exhaust gases withhydrogen by a catalytic Water gas shift reaction to produce a hydrogenenriched gas, diluting the resulting exhaust gases with gaseous oxygento produce lean conditions, trapping hydrocarbons in a hydrocarbon trap,and initiating oxidation of the hydrogen and CO over a three-waycatalyst.
 10. A methodlaccording to claim 9, wherein during the start-upportion of the engine operating cycle, the quantity of hydrogen in thehydrogen enriched gas is at least 5% by volume.
 11. A method accordingto claim 10, wherein the quantity of hydrogen is from 5 to 7% by volume.